Abstract. The overall goal of this proposal is to identify the mechanisms by which IFN1 and IL-17 act in a coordinated fashion to initiate the development of autoantibody (autoAb)-forming B cells in both human and mouse that develop systemic lupus erythematosus (lupus or SLE). Both IL-17R and IFN1R signaling have been implicated in the formation of spontaneous germinal centers (GCs) in the BXD2 mouse model of lupus through analysis of IFN1R-deficient and IL-17R-deficient BXD2 mice. There are high numbers of plasmacytoid DCs in the marginal sinus of the BXD2 mice that produce locally high levels of IFN1. IFN1 appears to influence the formation of GCs by promoting the migration of a population of CD21hiIgMhiCD23hi B cells from the marginal zone (MZ) to the follicle (FO). These B cells were referred to as CD23hiAPChi B cells as they have a very high capacity for antigen (Ag) presentation and the ability to induce a much stronger TH-17 polarization response than the conventional FO B cells. Parallel analyses indicate that IL-17 promotes the formation and stabilization of the GCs in the BXD2 mice through upregulation of regulators of G-protein signaling (Rgs)13 and Rgs16 in the B cells, which then promote the retention of APChi B cells in the FO area. We hypothesize that IFNa facilitates the release of the CD23hiAPChi B cells from the MZ and that IL-17 then localizes these cells in the FO where the CD4 T cells reside. As these CD23hiAPChi B cells have increased APC and costimulatory functions, they promote the generation/maintenance of TH-17 cells and the formation of spontaneous GCs. Three major questions will be addressed: (1) Does the IFN1-induced influx of the CD23hiAPChi B cells initiate the development of GCs in BXD2 mice? (2) Do the CD23hiAPChi B cells respond to IL-17 and promote polarization of TH-17 cells leading to the production of autoAbs in the BXD2 mice? And (3) Can IFNa and IL-17 regulate the migration behavior of APChi B cells in lupus patients? We will analyze Ag capture, transportation and presentation by APChi B cells to CD4 T cells in vivo using confocal microscopy and FACS analysis. The ability of IL-17 and IFNa to regulate chemotaxis will be determined in vitro, and confirmed in vivo by analysis of homing of GFP-positive APChi B cells using B cells from BXD2-Il-17r-/-, BXD2-Ifnar-/-, BXD2-Rgs13-/- and BXD2-Rgs16-/- mice. The clinical relevance of this model of the development of autoimmunity will be assessed by that analysis of B cells obtained from the peripheral blood of lupus patients for the analysis of the effects of IFN1 and IL-17 on the development and migration of APChi B cells. SIGNIFICANCE: These studies should identify critical migratory signals and upstream mediators that facilitate the development of an autoimmune circuit that leads to the production of high affinity pathogenic autoAbs. Thus, they have the potential to identify novel candidate therapeutic targets and to suggest more effective therapeutic interventions in autoAb-mediated autoimmune diseases.